5. GALACTIC CORONAS

Results of these calculations were reported at the First European
Astronomy Meeting by
Einasto
(1974).
The main conclusion was:
it is impossible to reproduce the rotation data by known stellar
populations only. The only way to eliminate the conflict between
photometric and rotational data was to assume the presence of an
unknown almost spherical population with a very high value of the
mass-to-light ratio, large radius and mass. To avoid confusion
with the conventional stellar halo, the term "corona" was suggested
for the massive population. Thus, the detailed modelling confirmed
earlier results obtained by simpler models. But here we have one
serious difficulty - no known stellar population has so large a M
/ L value.

Additional arguments for the presence of a spherical massive
population in spiral galaxies came from the stability criteria against
bar formation, suggested by
Ostriker &
Peebles (1973).
Their numerical
calculations demonstrated that flat systems become rapidly thicker and
evolve to a bar-like body. In real spiral galaxies a thin population
exists, and it has no bar-like form. To remain stable galaxies must
have a massive spherical halo.

The rotation data available in the early 1970s allowed the determination
the mass distribution in galaxies up to their visible edges. In order
to find how large and massive galactic coronas or halos are, more
distant test particles are needed. If halos are large enough, then in
pairs of galaxies the companion galaxies are located inside the halo,
and their relative velocities can be used instead of the galaxy
rotation velocities to find the distribution of mass around giant
galaxies. This test was made by
Einasto et
al. (1974).
A similar study was made independently by
Ostriker et
al. (1974).
Our results
were first discussed in the Caucasus Winter School on Cosmology in January
1974 and in the Tallinn Conference on Dark Matter in January 1975
(Doroshkevich
et al. 1975).

The mass of galactic coronas exceeds the mass of populations of known
stars by one order of magnitude. According to new estimates the total
mass density of matter in galaxies is 20% of the critical
cosmological density. The data suggest that all giant galaxies have
massive halos/coronas, thus dark matter must be the dynamically
dominating population in the whole Universe.

Initially the presence of massive coronas/halos was met with
scepticism. In the Third European Astronomical Meeting
the principal discussion was between the supporters of the classical
paradigm with conventional mass estimates of galaxies, and of the new
one with dark matter. The major arguments supporting the classical
paradigm were summarised by
Materne &
Tammann (1976).
Their most serious argument was:

"Big Bang nucleosynthesis suggests a low-density
Universe with the density parameter
0.05; the
smoothness of the Hubble flow also favours a low-density Universe."

Additional observational data gave strong support to the presence of
massive coronas/halos. Available rotation data were summarised by
Roberts (1975).
Extended rotation curves were available for 14
galaxies. In all galaxies the local mass-to-light ratio in the
periphery reached values over 100 in solar units. Rubin et al.
(1978,
1980)
measured optically the rotation curves of galaxies at
very large galactocentric distances.
Bosma (1978)
measured rotation data for 25 spiral galaxies with the Westerbork Synthesis
Radio Telescope. Both results suggested that practically all spiral
galaxies have extended flat rotation curves.

Another very important measurement was made by
Faber &
Jackson (1976),
Faber et
al. (1977),
Faber &
Gallagher (1979).
They measured the central velocity dispersions for 25 elliptical
galaxies and the rotation velocity of the Sombrero galaxy, just outside
the main body of the bulge. Their data yielded for the bulge of the
Sombrero galaxy a mass-to-light ratio M/L = 3, and for the
mean mass-to-light ratios for elliptical
galaxies about 7. These results showed that the mass-to-light ratios
of stellar populations in spiral and elliptical galaxies are similar
for a given colour, and the ratios are much lower than accepted in
earlier studies based on the dynamics of groups and clusters. In
other words, high mass-to-light ratios of groups and clusters of
galaxies cannot be explained by visible galactic populations.

The distribution of the mass in clusters can be determined if the
density and the temperature of the intra-cluster gas are known. These
data can be measured by the Einstein X-ray orbiting observatory. The
mass of Coma, Perseus and Virgo clusters was calculated from X-ray
data by
Bahcall &
Sarazin (1977),
Mathews
(1978).
The results confirmed previous estimates of masses made with the virial
method using galaxies as test particles.

Finally, masses of clusters of galaxies can be measured using
gravitational lensing of distant galaxies by clusters. The masses of
clusters of galaxies determined using this method, confirm the results
obtained by the virial theorem and the X-ray data
(Fischer &
Tyson 1997,
Fischer et
al. 1997).

Earlier suggestions on the presence of mass discrepancy in galaxies
and galaxy systems had been ignored by the astronomical community.
This time new results were taken seriously. However, it was still not
clear how to explain the controversy of the Big Bang nucleosynthesis
and the smoothness of the Hubble flow, discussed by
Materne &
Tammann (1976).